Propylene polymer with improved processability in thermoforming

ABSTRACT

The present invention concerns a propylene polymer comprising at least two propylene polymer fractions of different melt flow index and a minor amount of at least one comonomer, said propylene polymer being further characterized by specific ranges for melt flow index, xylene solubles content and recovery compliance. Said propylene polymer is particularly suited for thermoforming. The present invention further concerns a process for producing said propylene polymer as well as its use in thermoforming.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2010/064556, filed Sep. 30,2010, which claims priority from EP 09171954.2, filed Oct. 1, 2009.

FIELD OF THE INVENTION

The present invention concerns a propylene polymer comprising at leasttwo propylene polymer fractions of different melt flow index and a minoramount of at least one comonomer, said propylene polymer being furthercharacterized by specific ranges for melt flow index, xylene solublescontent and recovery compliance. Said propylene polymer is particularlysuited for thermoforming. The present invention further concerns aprocess for producing said propylene polymer as well as its use inthermoforming.

THE TECHNICAL PROBLEM AND THE PRIOR ART

In thermoforming, a soft polymer sheet is draped over or into a form ormold. In its basic form a thermoforming process comprises the steps of

-   -   (i) warming the sheet to a temperature at which it is soft,    -   (ii) draping the soft sheet over or into a mold, thus obtaining        a formed sheet,    -   (iii) cooling the formed sheet to a temperature at which it can        maintain its shape, and    -   (iv) removing the formed sheet from the mold.

In contrast to other forming processes, such as for example injectionmolding or blow molding, thermoforming is a low-pressure andlow-temperature process.

Generally the polymer sheet serving as feedstock for the thermoformingprocess is produced by melt-extrusion. Thus, one often speaks of“extrusion-thermoforming”to denote the complete process with the twodistinct processing stages of

-   -   (i) the production of a sheet by melt-extrusion of a polymer,        and    -   (ii) the thermoforming stage, wherein the sheet is formed or        shaped.

Extrusion-thermoforming comes in numerous variations. It may for examplebe done either in-line, i.e. the sheet is directly fed from thesheet-extrusion step to the thermoforming step, or off-line, i.e. thesheet is stored before being fed to the thermoforming step.

The polymers used in thermoforming are in most cases thermoplasticpolymers, which may be distinguished into amorphous and crystalline orsemi-crystalline polymers. Amorphous polymers are characterized in thatthey become progressively softer as temperature increases. Examples ofsuch polymers are polystyrene and polycarbonate. Semi-crystalline orcrystalline polymers by contrast are characterized by melting at aspecific temperature, around which within a few degrees they become toofluid for thermoforming. Examples are polypropylene and polyethylene.

Historically, amorphous polymers are preferred in thermoforming becausethey have a broader processing window than (semi-)crystalline polymers.Polystyrene, for example, can be thermoformed at a temperature between127° C. and 180° C., i.e. it has a processing window of more than 50° C.By contrast, polypropylene becomes too fluid above its meltingtemperature and hence generally is thermoformed at a temperature, whichmust be below the melting point but sufficiently high to bethermoformable. Polypropylene's processing window therefore is onlyabout 3° C. (see J. L. Throne, Understanding Thermoforming, Carl HanserVerlag, Munich, 1999, page 12).

Polypropylene, however, is of great interest because it offers goodmechanical and chemical properties in combination with good economics.Thermoforming companies as well as polymer producers have thereforeundertaken major research and development efforts to renderpolypropylene more suitable for use in thermoforming. So far, however,all efforts to broaden the processing window of polypropylene inthermoforming have been unsuccessful.

There still remains a need in the industry for polypropylenes withimproved processability in thermoforming, preferably without sacrificingother properties, such as for example processability in themelt-extrusion step or mechanical properties of the final thermoformedarticles.

It is therefore an object of the present invention to provide apropylene polymer that is suited for thermoforming.

It is also an object of the present invention to provide a propylenepolymer with improved processability in thermoforming.

it is a further object of the present invention to provide a propylenepolymer with good optical properties.

Furthermore, it is an object of the present invention to provide apropylene polymer with good mechanical properties.

In addition, it is an object of the present invention to provide apropylene polymer having good mechanical and optical properties incombination with good processability, particularly processability inthermoforming.

BRIEF DESCRIPTION OF THE INVENTION

We have now discovered that any of these objectives, either by itself ofin any combination, can be met by providing a propylene polymercomprising at least two propylene polymer fractions of different meltflow index and a minor amount of at least one comonomer, said propylenepolymer being further characterized by specific ranges for melt flowindex, xylene solubles content and recovery compliance.

Thus, the present invention provides a propylene polymer comprising atleast one comonomer and at least two propylene polymer fractions ofdifferent melt flow index, said propylene polymer being characterized by

-   -   a melt flow index in the range from 0.5 dg/min to 8.0 dg/min,        determined according to ISO 1133, condition L at 230° C. and        2.16 kg,    -   a xylene solubles content in the range from 1.0 wt % to 4.0 wt        %, relative to the total weight of the propylene polymer, and    -   a recovery compliance of at least 6.0·10⁻⁴ Pa⁻¹ and of at most        7.5·10⁻⁴ Pa⁻¹,    -   a total comonomer content of from 0.1 wt % to 1.0 wt %, relative        to the total weight of the propylene polymer,        wherein the fraction with the lowest melt flow index has a melt        flow index in the range from 0.2 dg/min to 1.0 dg/min,        determined according to ISO 1133, condition L at 230° C. and        2.16 kg.

The present invention also provides a process for the production of thepropylene polymer of the present invention in presence of

-   -   (i) at least two Ziegler-Natta polymerization catalysts having        different internal donors, each Ziegler-Natta polymerization        catalyst having a different hydrogen response, each of said        Ziegler-Natta catalysts comprising a titanium compound, which        has at least one titanium-halogen bond, and an internal donor,        both supported on magnesium halide in active form,    -   (ii) an organoaluminium compound (Al),    -   (iii) an external electron donor (ED), and    -   (iv) hydrogen,        said process comprising the step of polymerizing propylene and        at least one comonomer in a single polymerization reactor, so as        to produce the propylene polymer of the present invention.

The present invention also provides a process for the production of thepropylene polymer of the present invention in at least two sequentialpolymerization reactors in presence of

-   -   (1) a Ziegler-Natta polymerization catalyst, said Ziegler-Natta        catalyst comprising a titanium compound, which has at least one        titanium-halogen bond, and an internal donor, both supported on        magnesium halide in active form,    -   (ii) an organoaluminium compound (Al),    -   (iii) an external electron donor (ED), and    -   (iv) hydrogen,        said process comprising the steps of

-   (a) polymerizing propylene or polymerizing propylene and at least    one comonomer in a first polymerization reactor to produce a first    propylene polymer fraction;

-   (b) transferring said first propylene polymer fraction to a second    polymerization reactor; and

-   (c) polymerizing propylene or polymerizing propylene and at least    one comonomer in said second polymerization reactor to produce a    second propylene polymer fraction,

-   (f) recovering said propylene polymer after the last of the    polymerization reactors.    wherein the hydrogen concentration in at least one of the sequential    polymerization reactors is different from the hydrogen concentration    in the remaining polymerization reactors, and wherein the hydrogen    concentration in at least one of the polymerization reactors is    controlled such as to produce therein the propylene polymer fraction    with the lowest melt flow index as defined above, so as to produce    the propylene polymer of claims 1 to 10.

Further, the present invention provides thermoformed articles producedwith the propylene polymer of the present invention as well as a processto produce such thermoformed articles.

Additionally, the present invention provides for the use of thepropylene polymer of the present invention in thermoforming forbroadening the thermoforming window by at least 0.5° C. as compared to apropylene homopolymer with a single propylene homopolymer fraction and arecovery compliance of 5.6·10⁻⁴ Pa⁻¹ thermoformed under essentially thesame conditions.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the present patent application the melt flow index isdetermined according to ISO 1133, condition L, at 230° C. and 2.16 kg.

Throughout the present application the terms “polypropylene” and“propylene polymer” may be used synonymously.

Throughout the present application the terms “forming” and “shaping” maybe used synonymously.

For the purposes of the present invention “sheet” is defined as having athickness in the range from 500 μm to 2000 μm.

To be suited for thermoforming the propylene polymer of the presentinvention has a melt flow index in the range from 0.5 dg/min to 8.0dg/min. Preferably the melt flow index is in the range from 1.0 dg/minto 5.0 dg/min, more preferably in the range from 1.5 dg/min to 4.5dg/min, even more preferably in the range from 2.0 dg/min to 4.0 dg/min,and most preferably in the range from 2.5 dg/min to 3.5 dg/min.

The propylene polymer of the present invention is further characterizedin that the xylene solubles content is in the range from 1.0 wt % to 4.0wt %, relative to the total weight of the propylene polymer. Preferably,the xylene solubles content is at least 1.3 wt %, relative to the totalweight of the propylene polymer.

Preferably, the xylene solubles content is at most 3.5 wt %, morepreferably at most 3.0 wt %, even more preferably at most 2.8 wt %, andmost preferably at most 2.5 wt %, relative to the total weight of thepropylene polymer. The xylene solubles content of the propylene polymeris determined by dissolving the propylene polymer in refluxing xylene,cooling the solution to 25° C., filtering the solution, and subsequentlyevaporating the solvent. The residue, which is the xylene solublefraction of the propylene polymer, is then dried and weighed. A moredetailed description of the method to determine the xylene solublescontent is given in the examples.

The propylene polymer of the present invention is also characterized inthat the recovery compliance is at least 6.0·10⁻⁴ Pa⁻¹ and at most7.5·10⁻⁴ Pa⁻¹. Preferably, said recovery compliance is at least 6.2·10⁻⁴Pa⁻¹, more preferably at least 6.4·10⁻⁴ Pa⁻¹, even more preferably atleast 6.5·10⁻⁴ Pa⁻¹, and most preferably at least 6.6·10⁻⁴ Pa⁻¹.Preferably said recovery compliance is at most 7.3·10⁻⁴ Pa⁻¹, morepreferably at most 7.1·10⁻⁴ Pa⁻¹, even more preferably at most 6.9·10⁻⁴Pa⁻¹, and most preferably at most 6.8×10⁻⁴ Pa⁻¹. The recovery complianceis determined at 230° C. using a parallel-plate rotational stressrheometer. The recovery compliance is defined as the recoverable straindivided by the stress applied during the test. A more detaileddescription of the test method is given in the examples.

For the propylene polymer of the present invention it is essential thatit comprises at least one comonomer and that the total comonomer contentof the propylene polymer of the present invention is in the range from0.1 wt % to 1.0 wt %, relative to the total weight of the propylenepolymer. Thus, the propylene polymer of the present invention is arandom copolymer of propylene and at least one comonomer. Preferably thetotal comonomer content is in the range from 0.2 wt % to 0.8 wt %, andmost preferably in the range from 0.3 wt % to 0.5 wt %, relative to thetotal weight of the propylene polymer. The total comonomer content canfor example be determined by analytical methods, such as IR- orNMR-analysis as described in more detail in the examples.

While the nature of the comonomer is not so important as long as it canbe copolymerized with propylene in presence of propylene polymerizationcatalysts, it is nevertheless preferred that the comonomer is analpha-olefin different from propylene. Examples of suitablealpha-olefins are ethylene, butene-1, pentene-1, hexene-1,4-methylene-pentene-1 and octene-1. Of these, ethylene, butene-1 andhexene-1 are preferred. Ethylene is the most preferred comonomer. Thus,the most preferred propylene polymer is a random copolymer of propyleneand ethylene.

During the polymerization reaction comonomer(s) can be introduced intothe growing polymer chains in blocks, i.e. a large number of comonomerunits following each other; or, alternatively, comonomer(s) can beintroduced in an essentially statistical distribution, i.e. the numberof comonomer units following each other is very limited. In the idealcase for an essentially statistical distribution isolated comonomerunits are interspersed between propylene monomer units.

In the propylene polymer of the present invention it is preferred thatat least 60 mol %, more preferably at least 70 mol % and most preferablyat least 80 mol % of the total amount of comonomer in the propylenepolymer is present as single comonomer units in the polymer chains ofthe propylene polymer. The amount of single comonomer units can bedetermined by NMR analysis according to the method described by G. J.Ray et al. in Macromolecules, vol. 10, n° 4, 1977, p. 773-778.

It is essential that the propylene polymer of the present inventioncomprises at least two propylene polymer fractions of different meltflow index, wherein the melt flow index of the propylene polymerfraction with the lowest melt flow index has a melt flow index in therange from 0.2 dg/min to 1.0 dg/min. Preferably said melt flow index ofthe propylene polymer fraction with the lowest melt flow index is in therange from 0.3 dg/min to 0.9 dg/min, more preferably in the range from0.4 dg/min to 0.8 dg/min and most preferably in the range from 0.5dg/min to 0.7 dg/min. Preferably the propylene polymer of the presentinvention comprises two, three or four propylene polymer fractions ofdifferent melt flow index, more preferably it comprises two or threepropylene polymer fractions of different melt flow index, and mostpreferably it comprises two propylene polymer fractions of differentmelt flow index.

It is preferred that the propylene polymer of the present inventioncomprises from 50 wt % to 70 wt %, most preferably from 55 wt % to 65 wt%, relative to the total weight of the propylene polymer, of saidpropylene polymer fraction with the lowest melt flow index.

With respect to the distribution of the comonomer in the propylenepolymer fractions of different melt flow index it is neverthelesspreferred that either each of the at least two propylene polymerfractions has substantially the same comonomer content, or that thepropylene polymer fraction with the lowest melt flow index contains atleast 80 wt % of the total comonomer content of the propylene polymer.

In the case that each of the at least two propylene polymer fractionshas substantially the same comonomer content, it is more preferred thatrelative to the comonomer content of the propylene polymer fraction withthe lowest melt flow index, the comonomer content of the other propylenepolymer fractions is from 70% to 130%, even more preferably from 80% to120%, still even more preferably from 90% to 110%, and most preferablyfrom 95% to 105%, under the provision that each of the at least twopropylene polymer fractions comprises comonomer. For example, if thecomonomer content of the propylene polymer fraction with the lowest meltflow index is 0.5 wt %, relative to the total weight of said propylenepolymer fraction, then 110% in an other propylene polymer fraction wouldcorrespond to a comonomer content of 0.55 wt %, relative to the totalweight of that respective other propylene polymer fraction.

In the case that the propylene polymer fraction with the lowest meltflow index contains at least 80 wt % of the total comonomer content ofthe propylene polymer, it is most preferred that the propylene polymerfraction with the lowest melt flow index contains at least 80 wt % andat most 95 wt % of the total comonomer content of the propylene polymer.

Preferably, the propylene polymer of the present invention is furthercharacterized by a high isotacticity, for which the content of mmmmpentads is a measure. The content of mmmm pentads is preferably in therange from 97.0% to 99.0%. The content of mmmm pentads is determined onthe heptane insoluble fraction of the xylene insoluble fraction by NMRanalysis according to the method described by G. J. Ray et al. inMacromolecules, vol. 10, n° 4, 1977, p. 773-778.

The propylene polymer of the present invention may contain additives,such as by way of example, antioxidants, light stabilizers, acidscavengers, lubricants, antistatic additives, nucleating/clarifyingagents, and colorants. An overview of such additives may be found inPlastics Additives Handbook, ed. H. Zweifel, 5^(th) edition, 2001,Hanser Publishers.

Preferably, the heterophasic propylene copolymers may contain one ormore nucleating agents. The nucleating agent used in the presentinvention can be any of the nucleating agents known to the skilledperson. It is, however, preferred that the nucleating agent be selectedfrom the group consisting of talc, carboxylate salts, sorbitol acetals,phosphate ester salts, substituted benzene tricarboxamides and polymericnucleating agents, as well as blends of these. The most preferrednucleating agents are talc, carboxylate salts, and phosphate estersalts.

The carboxylate salts used as nucleating agents in the present inventioncan be organocarboxylic acid salts. Particular examples are sodiumbenzoate and lithium benzoate. The organocarboxylic acid salts may alsobe alicyclic organocarboxylic acid salts, preferably bicyclicorganodicarboxylic acid salts and more preferably abicyclo[2.2.1]heptane dicarboxylic acid salt or acyclohexanedicarboxylic acid salt. Nucleating agents of this type aresold as HYPERFORM® HPN-68 resp. HYPERFORM® HPN-20E by Milliken Chemical.

Examples for sorbitol acetals are dibenzylidene sorbitol (DBS),bis(p-methyl-dibenzylidene sorbitol) (MDBS), bis(p-ethyl-dibenzylidenesorbitol), bis(3,4-dimethyl-dibenzylidene sorbitol) (DMDBS), andbis(4-propylbenzylidene)propyl sorbitol. Bis(3,4-dimethyl-dibenzylidenesorbitol) (DMDBS) is preferred. These sorbitols can for example beobtained from Milliken Chemical under the trade names of Millad 3905,Millad 3940, Millad 3988, and Millad NX 8000.

Examples of phosphate ester salts are salts of2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate. Such phosphateester salts are for example available as NA-11 or NA-21 from AsahiDenka.

Examples of substituted tricarboxamides are those of general formula

wherein R1, R2 and R3, independently of one another, are selected fromC₁-C₂₀ alkyls, C₅-C₁₂ cycloalkyls, or phenyl, each of which may in turnby substituted with C₁-C₂₀ alkyls, C₅-C₁₂ cycloalkyls, phenyl, hydroxyl,C₁-C₂₀ alkylamino or C₁-C₂₀ alkyloxy etc. Examples for C₁-C₂₀ alkyls aremethyl, ethyl, n-propyl, n-butyl, iso-butyl, tert-butyl, n-pentyl,iso-pentyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 3-methylbutyl,hexyl, heptyl, octyl or 1,1,3,3-tetramethylbutyl. Examples for C₅-C₁₂cycloalkyl are cyclopentyl, cyclohexyl, cyclooctyl, cyclododecyl,adamantyl, 2-methylcyclohexyl, 3-methylcyclohexyl or2,3-dimethylcyclohexyl. Such nucleating agents are disclosed in WO03/102069 and by Blomenhofer et al. in Macromolecules 2005, 38,3688-3695.

Examples of polymeric nucleating agents are polymeric nucleating agentscontaining vinyl compounds, which are for example disclosed inEP-A1-0152701 and EP-A2-0368577. The polymeric nucleating agentscontaining vinyl compounds can either be physically or chemicallyblended with the polypropylene. In physical blending the polymericnucleating agent containing vinyl compounds is mixed with thepolypropylene in an extruder or in a blender. In chemical blending thepolypropylene comprising the polymeric nucleating agent containing vinylcompounds is produced in a polymerization process having at least twostages, in one of which the polymeric nucleating agent containing vinylcompounds is produced. Preferred vinyl compounds are vinyl cycloalkanesor vinyl cycloalkenes having at least 6 carbon atoms, such as forexample vinyl cyclopentane, vinyl-3-methyl cyclopentane, vinylcyclohexane, vinyl-2-methyl cyclohexane, vinyl-3-methyl cyclohexane,vinyl norbornane, vinyl cylcopentene, vinyl cyclohexene, vinyl-2-methylcyclohexene. The most preferred vinyl compounds are vinyl cyclopentane,vinyl cyclohexane, vinyl cyclopentene and vinyl cyclohexene.

Further, it is possible to use blends of nucleating agents, such as forexample a blend of talc and a phosphate ester salt or a blend of talcand a polymeric nucleating agent containing vinyl compounds.

While it is clear to the skilled person that the amount of nucleatingagent to be added depends upon its crystallization efficiency, for thepurposes of the present invention the nucleating agent or the blend ofnucleating agents is present in the polypropylene in an amount of atleast 50 ppm, preferably at least 100 ppm. It is present in an amount ofat most 11000 ppm, preferably of at most 5000 ppm, more preferably of atmost 4000 ppm, even more preferably of at most 3000 ppm and mostpreferably of at most 2000 ppm.

The propylene polymer of the present invention as defined above isproduced by polymerizing propylene and at least one comonomer inpresence of a Ziegler-Natta polymerization catalyst, an organoaluminiumcompound, an external electron donor (ED) and hydrogen.

A Ziegler-Natta polymerization catalyst comprises a titanium component,which has at least one titanium-halogen bond, and an internal donor,both supported on magnesium halide in active form.

The internal donor used in the present invention is a compound selectedfrom the group consisting of phthalates, diethers, succinates,di-ketones, enamino-amines and any blend of these. Alternatively to ablend of internal donors in a single Ziegler-Natta polymerizationcatalyst, it is also possible to employ a respective blend ofZiegler-Natta polymerization catalysts, wherein each of the catalystscomprises a single internal donor. The preferred internal donor is acompound selected from the group consisting of phthalates, diethers,succinates and any blend of these. The most preferred internal donor isa compound selected from the group consisting of phthalates, diethers orblends of these.

Suitable phthalates are selected from the alkyl, cycloalkyl and arylphthalates, such as for example diethyl phthalate, diisobutyl phthalate,di-n-butyl phthalate, dioctyl phthalate, diphenyl phthalate andbenzylbutyl phthalate. Such catalysts are for example commerciallyavailable from Basell under the Avant trade name.

Suitable diethers are 1,3-diethers of formulaR¹R²C(CH₂OR³)(CH₂OR⁴)wherein R¹ and R² are the same or different and are C₁-C₁₈ alkyl, C₃-C₁₈cycloalkyl or C₇-C₁₈ aryl radicals; R³ and R⁴ are the same or differentand are C₁-C₄ alkyl radicals; or are the 1,3-diethers in which thecarbon atom in position 2 belongs to a cyclic or polycyclic structuremade up of 5, 6 or 7 carbon atoms and containing two or threeunsaturations. Ethers of this type are disclosed in published Europeanpatent applications EP-A-0 361 493 and EP-A-0 728 769. Representativeexamples of said diethers are 2-methyl-2-isopropyl-1,3-dimethoxypropane;2,2-diisobutyl-1,3-dimethoxypropane;2-isopropyl-2-cyclo-pentyl-1,3-dimethoxypropane;2-isopropyl-2-isoamyl-1,3-dimethoxypropane;9,9-bis(methoxymethyl)fluorene.

Suitable succinate compounds have the formula

wherein R¹ to R⁴ are equal to or different from one another and arehydrogen, or a C₁-C₂₀ linear or branched alkyl, alkenyl, cycloalkyl,aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms,and R¹ to R⁴, being joined to the same carbon atom, can be linkedtogether to form a cycle; and R⁵ and R⁶ are equal to or different fromone another and are a linear or branched alkyl, alkenyl, cycloalkyl,aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms.

Suitable di-ketones are 1,3-di-ketones of formula

wherein R² and R³ are equal to or different from one another and arehydrogen, or a C₁-C₂₀ linear or branched alkyl, alkenyl, cycloalkyl,aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms,and R² and R³, being joined to the same carbon atom, can be linkedtogether to form a cycle; and R¹ and R⁴ are equal to or different fromone another and are a linear or branched alkyl, alkenyl, cycloalkyl,aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms.

Suitable enamino-imines have the general formula

wherein R² and R³ are equal to or different from one another and arehydrogen, or a C₁-C₂₀ linear or branched alkyl, alkenyl, cycloalkyl,aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms,and R² and R³, being joined to the same carbon atom, can be linkedtogether to form a cycle; and R¹ and R⁴ are equal to or different fromone another and are a linear or branched alkyl, alkenyl, cycloalkyl,aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms.

The organoaluminium compound is advantageously an alkyl-aluminiumcompound of the trialkyl-aluminium family, such as triethyl-aluminium,triisobutyl-aluminium, tri-n-butyl-aluminium, and linear or cyclicalkyl-aluminium compounds containing two or more Al atoms bonded to eachother by way of O or N atoms, or SO₄ or SO₃ groups. Triethyl-aluminiumis preferred. Advantageously, the trialkyl-aluminium has a hydridecontent, expressed as AlH₃, of less than to wt % with respect to thetrialkyl-aluminium. More preferably, the hydride content is less than0.5 wt %, and most preferably the hydride content is less than 0.1 wt %.The organoaluminium compound is used in such an amount as to have amolar ratio AIM in the range from 1 to 1000.

Suitable external electron donors (ED) include certain silanes, ethers,esters, amines, ketones, heterocyclic compounds and blends of these. Itis preferred to use a 1,3-diether as defined above or a silane. It ismost preferred to use a silane of the general formulaR^(a) _(p)R^(b) _(q)Si(OR^(c))_((4-p-q))wherein R^(a), R^(b) and R^(c) denote a hydrocarbon radical, inparticular an alkyl or cycloalkyl group, and wherein p and q are numbersranging from 0 to 3 with their sum p+q being equal to or less than 3.R^(a), R^(b) and R^(c) can be chosen independently from one another andcan be the same or different. Specific examples of such silanes are(tert-butyl)₂Si(OCH₃)₂, (cyclohexyl)(methyl) Si(OCH₃)₂ (referred to as“C donor”), (phenyl)₂Si(OCH₃)₂ and (cyclopentyl)₂ Si(OCH₃)₂ (referred toas “D donor”). (Cyclopentyl)₂ Si(OCH₃)₂ is the most preferred externaldonor.

The molar ratio of organo-aluminium compound to external donor (“Al/ED”)ranges between 1 and 1000. Said Al/ED molar ratio preferably is at most500, more preferably at most 200, even more preferably at most 100, andmost preferably at most 50. Said Al/ED molar ratio preferably is atleast 5. It is clear to the person skilled in the art that the molarratio of organo-aluminium compound to external donor (“Al/ED”) must bechosen such that the resulting propylene polymer has a xylene solublescontent as defined above. This is well within the capabilities of theskilled person and need not be further described.

Hydrogen is used to control the chain lengths of the propylene polymer.For the production of a propylene polymer with higher MFI, i.e. withlower average molecular weight and shorter polymer chains, theconcentration of hydrogen in the polymerization medium needs to beincreased. Inversely, the hydrogen concentration in the polymerizationmedium has to be reduced in order to produce a propylene polymer withlower MFI, i.e. with higher average molecular weight and longer polymerchains.

Before being fed to the first polymerization reactor the Ziegler-Nattapolymerization catalyst of the blend of Ziegler-Natta polymerizationcatalysts preferably undergoes a premix and/or a pre-polymerizationstep. In the premix step, the triethyl aluminium (TEAL) and the externalelectron donor (ED)—if present—, which have been pre-contacted, aremixed with the Ziegler-Natta catalyst at a temperature in the range from0° C. to 30° C., preferably in the range from 5° C. to 20° C., for up to15 min. The mixture of TEAL, external electron donor and Ziegler-Nattacatalyst is pre-polymerized with propylene at a temperature in the rangefrom 10° C. to 100° C., preferably in the range from 10° C. to 30° C.,for 1 to 30 min, preferably for 2 to 20 min.

The polymerization of propylene and the one or more comonomers iscarried out according to known techniques. The polymerization can forexample be carried out in liquid propylene as reaction medium. It canalso be carried out in a diluent, such as an inert hydrocarbon (slurrypolymerization) or in the gas phase. For the present invention thepropylene polymer is preferably produced by polymerization in liquidpropylene at temperatures in the range from 20° C. to 100° C.Preferably, temperatures are in the range from 60° C. to 80° C. Thepressure can be atmospheric or higher. Preferably the pressure isbetween 25 bar and 50 bar.

The propylene polymer of the present invention comprising at least twopropylene polymer fractions of different melt flow indices can beproduced

-   (I) in a single polymerization reactor in presence of at least two    Ziegler-Natta polymerization catalysts having different internal    donors, each Ziegler-Natta polymerization catalyst having a    different hydrogen response,-   or-   (II) in at least two sequential polymerization reactors in presence    of a single Ziegler-Natta polymerization catalyst, wherein the    hydrogen concentration in at least one of the polymerization    reactors is different from the hydrogen concentration(s) in the    remaining polymerization reactor(s).

Production method (II) is, however, preferred. The term “differenthydrogen response” is used to denote that under essentially identicalpolymerization conditions, particularly under essentially the samehydrogen concentration, different polymerization catalysts result inpropylene polymers having different melt flow indices.

In the case of a single polymerization reactor, the process for theproduction of the propylene polymer of the present invention in presenceof

-   -   (i) at least two Ziegler-Natta polymerization catalysts having        different internal donors, each Ziegler-Natta polymerization        catalyst having a different hydrogen response, each of said        Ziegler-Natta catalysts comprising a titanium compound, which        has at least one titanium-halogen bond, and an internal donor,        both supported on magnesium halide in active form,    -   (ii) an organoaluminium compound (Al),    -   (iii) an external electron donor (ED), and    -   (iv) hydrogen,        comprises the step of polymerizing propylene and at least one        comonomer in a single polymerization reactor, so as to produce        the propylene polymer of the present invention as defined above.

In the preferred case of at least two sequential polymerizationreactors, the process for the production of the propylene polymer of thepresent invention in presence of

-   -   (i) a Ziegler-Natta polymerization catalyst, said Ziegler-Natta        catalyst comprising a titanium compound, which has at least one        titanium-halogen bond, and an internal donor, both supported on        magnesium halide in active form,    -   (ii) an organoaluminium compound (Al),    -   (iii) an external electron donor (ED), and    -   (iv) hydrogen,        comprises the steps of

-   (a) polymerizing propylene or polymerizing propylene and at least    one comonomer in a first polymerization reactor to produce a first    propylene polymer fraction;

-   (b) transferring said first propylene polymer fraction to a second    polymerization reactor; and

-   (c) polymerizing propylene or polymerizing propylene and at least    one comonomer in said second polymerization reactor to produce a    second propylene polymer fraction,

-   (f) recovering said propylene polymer after the last of the    polymerization reactors.    wherein the hydrogen concentration in at least one of the sequential    polymerization reactors is different from the hydrogen concentration    in the remaining polymerization reactors, and wherein the hydrogen    concentration in at least one of the polymerization reactors is    controlled such as to produce therein the propylene polymer fraction    with the lowest melt flow index as defined above, so as to produce    the propylene polymer of the present invention as defined above.    With respect to step (a) it is preferred that said first propylene    polymer fraction is produced by polymerizing propylene and at least    one comonomer. With respect to step (c) it is preferred that said    second propylene polymer fraction is produced by polymerizing    propylene and at least one comonomer.

While the number of polymerization reactors is not limited, it isnevertheless preferred for practical reasons to use two, three or foursequential polymerization reactors, more preferably two or threesequential polymerization reactors and most preferably two sequentialpolymerization reactors. If said process for the production of thepropylene polymer of the present invention is performed in more than twosequential polymerization reactors, said process further comprises thesteps of

-   (d) transferring the combined propylene polymer fractions produced    in the previous polymerization reactors to a further polymerization    reactor; and-   (e) polymerizing propylene or polymerizing propylene and at least    one comonomer in said further polymerization reactor to produce a    further propylene polymer fraction.    Depending upon the number of polymerization reactors steps (d)    and (e) may be repeated. With respect to step (e) it is preferred    that said further propylene polymer fraction is produced by    polymerizing propylene and at least one comonomer.

In the case of at least two sequential polymerization reactors, thepropylene polymer fraction with the lowest melt flow index within theabove defined ranges, may be produced in any of the polymerizationreactors. For example, in the case of two polymerization reactors thepropylene polymer fraction with the lowest melt flow index may either beproduced in the first or the second polymerization reactor. It is,however, preferred that the propylene polymer fraction with the lowestmelt flow index is produced in the first polymerization reactor.

Under the provision that the total amount of the at least one comonomerof the propylene polymer and of the propylene polymer fractions of thepresent invention fails within the ranges as defined above, the at leastone comonomer may be introduced into the at least two sequentialpolymerization reactors in such a way that the propylene polymerfractions produced in each of these polymerization reactors containeither the same or different contents of the at least one comonomer, asgiven in wt % relative to the total weight of the respective propylenepolymer fraction. For the purposes of the present invention it ispreferred that the at least one comonomer is introduced into the atleast two sequential polymerization reactors in such a way that eitherall of the propylene polymer fractions of the propylene polymer have thesame comonomer content, as given in wt % relative to the total weight ofthe respective propylene polymer fraction, or that the comonomer contentof the propylene polymer fraction with the lowest melt flow index issuch that it contains at least 80 wt % of the total comonomer content ofthe propylene polymer.

In the case that the at least one comonomer is introduced into the atleast two sequential polymerization reactors in such a way that all ofthe propylene polymer fractions of the propylene polymer havesubstantially the same comonomer content, it is more preferred that theat least one comonomer is introduced in such a way that relative to thecomonomer content of the propylene polymer fraction with the lowest meltflow index, the comonomer content of the other propylene polymerfractions is from 70% to 130%, even more preferably from 80% to 120%,still even more preferably from 90% to 110%, and most preferably from95% to 105%, under the provision that the at least one comonomer isintroduced into each of the at least two sequential polymerizationreactors, i.e. that each of the propylene polymer fractions producedcomprises comonomer.

In the case that the at least one comonomer is introduced into the atleast two sequential polymerization reactors in such a way that thepropylene polymer fraction with the lowest melt flow index contains atleast 80 wt % of the total comonomer content of the propylene polymer,it is most preferred that the at least one comonomer is introduced insuch a way that the propylene polymer fraction with the lowest melt flowindex contains at least 80 wt % and at most 95 wt % of the totalcomonomer content of the propylene polymer.

In the case of two sequential polymerization reactors, the totalcomonomer content of the propylene polymer is either introduced into thefirst or the second or both polymerization reactors. If the totalcomonomer content of the propylene polymer is introduced into onereactor only, it is clear that the comonomer content of the propylenepolymer fraction produced therein needs to be adapted accordingly,taking account of the contribution of the respective polymerizationreactor to the total weight of the propylene polymer. It is, however,preferred that the comonomer is introduced in both reactors so as toproduce the propylene polymer and the respective propylene polymerfractions defined earlier in this application.

The propylene polymer of the present invention is used in the productionof thermoformed articles, particularly in the production of transparentthermoformed articles. Examples of such articles are food storagecontainers, drinking cups etc.

Thermoformed articles are generally produced by a two-stage process,wherein in the first stage a sheet is produced by melt-extruding apolymer, and in the second stage said sheet is shaped (thermoformingstage). The two stages may either directly follow each other (in-linethermoforming) or they may not directly follow each other, in which casethe produced sheet is stored first and only later fed to thethermoforming stage.

The sheet may be produced on any melt extrusion sheet line, theproduction process for example comprising the steps of

-   -   (I-a) feeding the propylene polymer of the present invention to        an extruder,    -   (I-b) melting the propylene polymer in the extruder,    -   (I-c) optionally passing the molten propylene polymer through a        melt pump,    -   (I-d) extruding the molten polymer through a slit die, and    -   (I-e) cooling the sheet.

The melt temperature of the propylene polymer generally is in the rangefrom 200° C. to 280° C., preferably in the range from 210° C. to 270°.As the process for producing sheet is well known to the skilled personno further description is deemed necessary. Exemplary sheet productionconditions are given in the examples.

The second stage, the thermoforming stage, can be done on anythermoforming machine comprising a heating and a forming section, saidthermoforming process comprising the steps of

-   -   (II-a) warming the sheet to a temperature at which it is soft,    -   (II-b) draping the soft sheet over or into a mold, thus        obtaining a formed sheet,    -   (II-c) cooling the formed sheet to a temperature at which it can        maintain its shape, and    -   (II-d) removing the formed sheet from the mold.

In the thermoforming stage the propylene polymer of the presentinvention can be processed under conditions that are comparable to theconditions used for a prior art propylene polymer.

The present inventors have noted with surprise that the use of thepropylene polymer of the present invention allows to broaden theprocessing window in the thermoforming stage as compared to prior artpolypropylene, such as a propylene homopolymer. Hence, the propylenepolymer of the present invention allows for easier processing in theforming step. At the same time the propylene polymer of the presentinvention has mechanical properties that are comparable to those of aprior art propylene homopolymer

In consequence, the present invention also discloses the use of thepropylene polymer as defined above in thermoforming for broadening thethermoforming window by at least 0.5° C. as compared to a propylenehomopolymer with a single propylene homopolymer fraction and a recoverycompliance of 5.6·10⁻⁴ Pa⁻¹ thermoformed under essentially the sameconditions. Preferably, the thermoforming window is broadened by atleast 0.6° C., more preferably by 0.8° C., even more preferably by atleast 1.0° C. and most preferably by at least 1.2° C.

EXAMPLES

The following examples illustrate the advantages of the presentinvention and also give exemplary processing conditions for the sheetproduction and the forming stages. It is deemed well within the skillsof the persons skilled in the art of thermoforming to adapt theseprocessing conditions to his/her specific equipment.

Test Methods

The melt flow index (MFI) was measured according to ISO 1133, conditionL, using a weight of 2.16 kg and a temperature of 230° C.

Flexural modulus was determined according to ISO 178:2001.

Notched izod impact strength was determined according to ISO 180/A:2000at 23° C.

Top load of the thermoformed cups was determined in accordance withISO12048:1994.

Haze was measured according to ISO 14782:1999 on injection moldedplaques having a thickness of 1 mm.

The total ethylene content (wt % C₂) relative to the total weight of thepropylene polymer is determined by NMR analysis of pellets according tothe method described by G. J. Ray et al. in Macromolecules, vol. 10, n°4, 1977, p. 773-778.

Xylene solubles (XS) were determined as follows: Between 4.5 and 5.5 gof propylene polymer were weighed into a flask and 300 ml xylene wereadded. The xylene was heated under stirring to reflux for 45 minutes.Stirring was continued for 15 minutes exactly without heating. The flaskwas then placed in a thermostated bath set to 25° C.+/−1° C. for 1 hour.The solution was filtered through Whatman n° 4 filter paper and exactly100 ml of solvent were collected. The solvent was then evaporated andthe residue dried and weighed. The percentage of xylene solubles (“XS”)was then calculated according toXS (in wt %)=(Weight of the residue/Initial total weight of PP)*300

The recovery compliance is determined at 230° C. using a parallel-platerotational stress rheometer. The sample is contained between two coaxialparallel discs in an oven filled with nitrogen. The test consists ofmonitoring the strain response when the stress has been deleted after acreep test. For the creep test a stress of 600 Pa is applied. Then therecovery compliance is the recoverable strain divided by the stressapplied during the creep.

The thermoforming window is determined as follows: Sheet having athickness of 1 mm is produced on a melt-extrusion line (see section onsheet extrusion below) and stored under ambient conditions for 7 days.The sheet is then thermoformed by plug-assisted pressure forming on aGabler Swing thermoforming machine into cups having a depth of 50 mm andan inner diameter of 85 mm at the top and 65-67 mm at the bottom with arim of 5 mm at the top using a four-fold mold, whereby only one of themolds is used for taking the samples for the determination of thethermoforming window. Initial oven settings for heating the sheet arechosen such that the sheet reaches a temperature at which it may just bethermoformed, and a total of 5 cups of the same mold are produced. Thesheet temperature is then increased in increments of 1° C., wherebyagain a total of 5 cups of the same mold is produced at eachtemperature, until the sheet can no longer be thermoformed. Thecollected cups are then subjected to a dynamic compression test todetermine the top load, the average of the top load of the 5 cupsproduced under identical conditions is taken and plotted against therespective sheet temperature. The plotted curve has a bell shape, i.e.the top load has a maximum. Said curve is then approximated by a 4^(th)degree polynomial equation of the general form Y=a·X⁴+b·X³+c·X²+d·X+e,which in the following is used in the determination of the thermoformingwindow. The thermoforming window is defined as the range in sheettemperatures at which the top load is at least 80% of the maximum topload determined for the respective sheet.

Propylene Polymers

The propylene polymers of the example (Ex. 1) and the comparativeexamples (Comp. ex. 1 to 3) were produced in an industrial propylenepolymerization plant having two sequential loop reactors. As catalyst acommercially available Ziegler-Natta polymerization catalyst with aphthalate as internal donor was used. As external donor, either(cyclohexyl)(methyl) Si(OCH₃)₂ (referred to as “C”) or (cyclopentyl)₂Si(OCH₃)₂ (referred to as “D”) were used as indicated in table 1,wherein n.a. is to denote the cases where data is not available. Furtherpolymerization conditions are given in table 1 as well, wherein theethylene content is given as wt % relative to the total weight of thepropylene polymer fraction produced in the respective loop. Propertiesof the so-obtained propylene polymers are given in table 2.

The so-obtained propylene polymers were additivated with antioxidantsand a nucleating agent in an amount sufficient to avoid excessivedegradation resp. to result in the desired level of transparency.

The melt flow index (MFI_(final)) of the propylene polymer produced inthe second polymerization reactor is calculated using the followingequationLog(MFI_(final))=w ₁·Log(MFI₁)+w ₂·Log(MFI₂)wherein MFI₁ and MFI₂ are the melt flow indices of the propylene polymerfractions produced in the first resp. the second polymerization reactor,and w₁ and w₂ are the respective weight fractions of the propylenepolymer fractions produced in the first resp. the second polymerizationreactor as expressed in wt % of the total propylene polymer produced inthe two polymerization reactors together. These weight fractions arecommonly also referred to as the contribution by the respectivepolymerization reactor.

More generally, the melt flow index (MFI) of the propylene polymer ofthe present invention can be calculated according to

${MFI}_{final} = {\sum\limits_{i}^{i = n}\;{w_{i} \cdot {{Log}\left( {MFI}_{i} \right)}}}$wherein w_(i) is the weight fraction of the respective propylene polymerfraction i as expressed in wt % of the total propylene polymer producedin all polymerization reactors, MFI_(i) is the melt flow index of therespective propylene polymer fraction i, and n is the number ofpropylene polymer fractions.

Overall ethylene content of the propylene polymer, abbreviated as %C2_(final), can be calculated according to% C ² _(final) =w ₁·% C2₁ +w ₂·% C2₂wherein % C2₁ and % C2₂ are the ethylene comonomer contents of thepropylene polymer fractions produced in the first resp. the secondpolymerization reactor, and w₁ and w₂ are the respective weightfractions of the propylene polymer fractions produced in the first resp.the second polymerization reactor as expressed in wt % of the totalpropylene polymer produced in the two polymerization reactors together.These weight fractions are commonly also referred to as the contributionby the respective polymerization reactor. Using this equation, theethylene content of the propylene polymer fraction produced in thesecond polymerization reactor can be calculated.

TABLE 1 Comp. Comp. Comp. Unit Ex. 1 ex. 1 ex. 2 ex. 3 CatalystPhthalate Phthalate Phthalate Phthalate External donor D C C D (ED)Catalyst activation TEAL/ g/kg 0.20 n.a. 0.20 0.16 Propylene TEAL/ED g/g3.5 n.a. 20 4 Loop 1 Temperature ° C. 70 71 72 70 Hydrogen vpm 470 n.a.210 460 Contribution % 59 61 53 57 Loop 1 Ethylene wt % 0.4 0 0 0content MFI₁ dg/min 0.6 3.0 0.7 0.7 Loop 2 Temperature ° C. 65 66 68.565 Hydrogen vpm 6020 n.a. 3300 5600 Ethylene wt % 0.4 0 0 0 content(calc.) MFI₂ (calc.) dg/min 42.5 3.0 18.3 29

TABLE 2 Comp. Comp. Comp. Unit Ex. 1 ex. 1 ex. 2 ex. 3 MFI dg/min 3.53.0 3.3 3.5 Xylene solubles wt % 2.3 4.0 3.3 1.7 Recovery compliance10⁻⁴ Pa⁻¹ 6.7 5.6 6.4 7.0 Comonomer content wt % 0.4 0 0 0 T_(melt) ° C.162 165 164 165 Flexural modulus MPa 1960 1700 1890 2100 Izod, notched,23° C. kJ/m² 5.0 5.5 5.5 4.1 Haze (1 mm) % 28 32 30 29

Sheet Extrusion

The propylene polymers of example 1 and comparative examples 1 to 3 wereextruded into 1 mm thick sheet on a 1 m wide Reifenhauser sheetextrusion line with an upward chill roll stack, a 70 mm extruder havinga ratio of length to diameter (L/D) of 33), a melt pump and a coathangerdie. Extrusion conditions are indicated in table 3.

TABLE 3 Extruder temperatures Zone 1 230° C. Zones 2-10 240° C. Dietemperatures 240° C. Chill roll temperatures Bottom 80° C. Middle 105°C. Top 105° C. Die gap 1200 μm Roll speed 3.5 m/min

Thermoforming

The so-obtained sheet was thermoformed as described above for thedetermination of the thermoforming window. Thermoforming conditions aregiven in table 4. Following the previously described procedure thethermoforming window can be determined. Table 4 also indicates as“lowest sheet temperature for forming” the lowest temperature at whichthermoformed cups were produced, i.e. the temperature at which the sheetmay just be thermoformed. Results for the thermoforming window and themaximum top load are indicated in table 5.

TABLE 4 Comp. Comp. Comp. Unit Ex. 1 ex. 1 ex. 2 ex. 3 Ambienttemperature ° C. 17-18 17-18 17-18 17-18 Cycle min⁻¹ 17.8 17.8 17.8 17.8Lowest sheet temp. ° C. 145 147 148 151 for forming Plug delay time s0.35 0.35 0.35 0.35 Air Delay time s 0.6 0.6 0.6 0.6 Pressure bar 4 4 44 Forming time s 1.5 1.5 1.5 1.5

TABLE 5 Unit Ex. 1 Comp. ex. 1 Comp. ex. 2 Comp. ex. 3 Max. top load N290 240 270 280 Width of ° C. 5.0 3.5 4.2 3.7 thermo-forming window

The present inventors have been very surprised to see that thethermoformed cups made with the propylene polymer of the presentinvention retain the mechanical properties of a propylene homopolymerbut at the same time have a significantly wider processing window.

Under industrial conditions the propylene polymer of example 1 has beenshown to allow a shortening of the thermoforming cycle time by 8% to 25%in the production of drinking cups, relative to the cycle time with thepropylene polymer of comparative examples 1 and 2 for the samethermoformed article.

The invention claimed is:
 1. A propylene polymer comprising at least onecomonomer and at least two propylene polymer fractions of different meltflow index, said propylene polymer being characterized by a melt flowindex ranging from 0.5 dg/min to 8.0 dg/min, determined according to ISO1133, condition L at 230° C. and 2.16 kg, a xylene solubles contentranging from 1.0 wt % to 4.0 wt %, relative to a total weight of thepropylene polymer, and a total comonomer content of from 0.1 wt % to 1.0wt %, relative to the total weight of the propylene polymer, wherein thefraction with the lowest melt flow index has a melt flow index rangingfrom 0.2 dg/min to 1.0 dg/min, determined according to ISO 1133,condition L at 230° C. and 2.16 kg; wherein each of the at least twopropylene polymer fractions has substantially the same comonomer contentand, relative to the comonomer content of the propylene polymer fractionwith the lowest melt flow index, the comonomer content of the otherpropylene polymer fraction is from 70% to 130%, under the provision thateach of the at least two propylene polymer fractions comprisescomonomer.
 2. The propylene polymer according to claim 1, wherein themelt flow index ranges from 1.0 dg/min to 5.0 dg/min, determinedaccording to ISO 1133, condition L, at 230° C. and 2.16 kg.
 3. Thepropylene polymer according to claim 1, wherein the total comonomercontent ranges from 0.2 wt % to 0.8 wt %, relative to the total weightof the propylene polymer.
 4. The propylene polymer according to claim 1,wherein the xylene solubles content is ranges from 1.3 wt % to 3.5 wt %,relative to the total weight of the propylene polymer.
 5. The propylenepolymer according to claim 1, wherein the fraction with the lowest meltflow index has a melt flow index ranging from 0.3 dg/min to 0.9 dg/min,determined according to ISO 1133, condition L, at 230° C. and 2.16 kg.6. The propylene polymer according to claim 1, wherein the propylenepolymer comprises from 50 wt % to 70 wt %, relative to the total weightof the propylene polymer, of the propylene polymer fraction with thelowest melt flow index.
 7. The propylene polymer according to claim 1,wherein each of the at least two propylene polymer fractions has thesame comonomer content.
 8. The propylene polymer according to claim 1,wherein at least 60 mol % of the total amount of comonomer is present assingle comonomer units.
 9. A thermoformed article comprising thepropylene polymer of claim
 1. 10. A process for the production of thepropylene polymer of claim 1 in presence of (i) at least twoZiegler-Natta polymerization catalysts having different internal donors,each Ziegler-Natta polymerization catalyst having a different hydrogenresponse, each of said Ziegler-Natta catalysts comprising a titaniumcompound, which has at least one titanium-halogen bond, and an internaldonor, both supported on magnesium halide in active form, (ii) an organoaluminium compound (Al), (iii) an external electron donor (ED), and (iv)hydrogen, said process comprising the step of polymerizing propylene andat least one comonomer in a single polymerization reactor, so as toproduce the propylene polymer of claim
 1. 11. A process for theproduction of the propylene polymer of claim 1 in at least twosequential polymerization reactors in presence of (i) a Ziegler-Nattapolymerization catalyst, said Ziegler-Natta catalyst comprising atitanium compound, which has at least one titanium-halogen bond, and aninternal donor, both supported on magnesium halide in active form, (ii)an organo aluminium compound (Al), (iii) an external electron donor(ED), and (iv) hydrogen, said process comprising the steps of (a)polymerizing propylene or polymerizing propylene and at least onecomonomer in a first polymerization reactor to produce a first propylenepolymer fraction; (b) transferring said first propylene polymer fractionto a second polymerization reactor; and (c) polymerizing propylene orpolymerizing propylene and at least one comonomer in said secondpolymerization reactor to produce a second propylene polymer fraction,(f) recovering said propylene polymer after the last of thepolymerization reactors; wherein the hydrogen concentration in at leastone of the sequential polymerization reactors is different from thehydrogen concentration in the remaining polymerization reactors, andwherein the hydrogen concentration in at least one of the polymerizationreactors is controlled such as to produce therein the propylene polymerfraction with the lowest melt flow index as defined above, so as toproduce the propylene polymer of claim 1.